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<!DOCTYPE html>
<html lang=en>
<head profile="http://www.w3.org/2006/03/hcard"><meta content="text/html;
charset=utf-8" http-equiv=Content-Type>
<title>CSS Variables Module Level 1</title>
<link href="http://dev.w3.org/csswg/css-module/default.css" rel=stylesheet
type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
type="text/css">
<style type="text/css">
/* delete this block when you've cleared out all the .replaceme elements */
.replaceme {
border: solid red;
padding: 0 0.5em;
margin: 0 0.1em;
background: yellow;
color: green;
}
.replaceme:before {
content: "FIXME(";
color: black;
}
.replaceme:after {
content: ")";
color: black;
}
</style>
<style>
pre.idl { border:solid thin; background:#eee; color:#000; padding:0.5em }
pre.idl :link, pre.idl :visited { color:inherit; background:transparent }
</style>
<body>
<div class=head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1>CSS Variables Module Level 1</h1>
<h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 30 September
2011</h2>
<dl>
<dt>This version:
<dd><a
href="http://www.w3.org/TR/2011/ED-css-variables-20110930/">http://dev.w3.org/csswg/css-variables/</a>
<!--<dt>Latest version:
<dd><a href="http://www.w3.org/TR/css-variables/">http://www.w3.org/TR/css-variables/</a>-->
<dt>Editor's draft:
<dd><a
href="http://dev.w3.org/csswg/css-variables/">http://dev.w3.org/csswg/css-variables/</a>
<dt>Editors:
<dd class=vcard> <a href="http://xanthir.com/contact/" rel=url> <span
class=fn>Tab Atkins Jr.</span>, </a> <span class=org>Google, Inc.</span>
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2011 <a
href="http://www.w3.org/"><acronym title="World Wide Web
Consortium">W3C</acronym></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute
of Technology">MIT</acronym></a>, <a href="http://www.ercim.eu/"><acronym
title="European Research Consortium for Informatics and
Mathematics">ERCIM</acronym></a>, <a
href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
and <a
href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a> rules apply.</p>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>Abstract</h2>
<p>CSS is a language for describing the rendering of structured documents
(such as HTML and XML) on screen, on paper, in speech, etc. This module
contains the features of CSS level 3 relating to <var
class=replaceme>DESCRIBE HERE</var>. It includes and extends the
functionality of CSS level 2 <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, which builds on CSS
level 1 <a href="#CSS1" rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>.
The main extensions compared to level 2 are <var
class=replaceme>SUMMARIZE HERE</var>.
<h2 class="no-num no-toc" id=status>Status of this document</h2>
<!--begin-status-->
<p>This is a public copy of the editors' draft. It is provided for
discussion only and may change at any moment. Its publication here does
not imply endorsement of its contents by W3C. Don't cite this document
other than as work in progress.
<p>The (<a
href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
mailing list <a
href="mailto:www-style@w3.org?Subject=%5Bcss-variables%5D%20PUT%20SUBJECT%20HERE">
www-style@w3.org</a> (see <a
href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
discussion of this specification. When sending e-mail, please put the text
“css-variables” in the subject, preferably like this:
“[<!---->css-variables<!---->] <em>…summary of
comment…</em>”
<p>This document was produced by the <a href="/Style/CSS/members">CSS
Working Group</a> (part of the <a href="/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
rel=disclosure>public list of any patent disclosures</a> made in
connection with the deliverables of the group; that page also includes
instructions for disclosing a patent. An individual who has actual
knowledge of a patent which the individual believes contains <a
href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
W3C Patent Policy</a>.</p>
<!--end-status-->
<p>The following features are at risk: …
<h2 class="no-num no-toc" id=contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#intro"><span class=secno>1. </span> Introduction</a>
<ul class=toc>
<li><a href="#placement"><span class=secno>1.1. </span> Module
Interactions</a>
<li><a href="#values"><span class=secno>1.2. </span> Values</a>
</ul>
<li><a href="#defining-variables"><span class=secno>2. </span> Defining
Variables</a>
<li><a href="#using-variables"><span class=secno>3. </span> Using
Variables</a>
<ul class=toc>
<li><a href="#using-invalid-variables"><span class=secno>3.1. </span>
Using Invalid Variables</a>
</ul>
<li><a href="#cssom"><span class=secno>4. </span> APIs</a>
<ul class=toc>
<li><a href="#cssom-simple"><span class=secno>4.1. </span> The Simple
API</a>
<li><a href="#cssom-stylesheet"><span class=secno>4.2. </span> Additions
to the Stylesheet Interface</a>
<ul class=toc>
<li><a href="#cssom-cssrule"><span class=secno>4.2.1. </span> Changes
to interface CSSRule</a>
<li><a href="#cssom-cssvariablerule"><span class=secno>4.2.2. </span>
Interface CSSVariableRule</a>
<li><a href="#cssom-cssvariable"><span class=secno>4.2.3. </span>
Interface CSSVariableComponentValue</a>
</ul>
</ul>
<li><a href="#grammar"><span class=secno>5. </span> The Grammar of
Variables</a>
<li><a href="#conformance"><span class=secno>6. </span> Conformance</a>
<ul class=toc>
<li><a href="#conventions"><span class=secno>6.1. </span> Document
Conventions</a>
<li><a href="#conformance-classes"><span class=secno>6.2. </span>
Conformance Classes</a>
<li><a href="#partial"><span class=secno>6.3. </span> Partial
Implementations</a>
<li><a href="#experimental"><span class=secno>6.4. </span> Experimental
Implementations</a>
<li><a href="#testing"><span class=secno>6.5. </span>Non-Experimental
Implementations</a>
<li><a href="#cr-exit-criteria"><span class=secno>6.6. </span> CR Exit
Criteria</a>
</ul>
<li class=no-num><a href="#acknowledgments">Acknowledgments</a>
<li class=no-num><a href="#references">References</a>
<ul class=toc>
<li class=no-num><a href="#normative-references">Normative
references</a>
<li class=no-num><a href="#other-references">Other references</a>
</ul>
<li class=no-num><a href="#index">Index</a>
<li class=no-num><a href="#property-index">Property index</a>
</ul>
<!--end-toc-->
<h2 id=intro><span class=secno>1. </span> Introduction</h2>
<p><em>This section is not normative.</em>
<p>Large documents or applications (and even small ones) can contain quite
a bit of CSS. Many of the values in the CSS file will be duplicate data;
for example, a site may establish a color scheme and reuse three or four
colors throughout the site. Altering this data can be difficult and
error-prone, since it's scattered throughout the CSS file (and possibly
across multiple files), and may not be amenable to Find-and-Replace. This
module introduces <b>Variables</b>, which allow a value to be assigned to
a name, which may then be used in place of the value elsewhere in the
document. This makes it easier to read large files, as seemingly-arbitrary
values now have informative names, and makes editting such files much
easier and less error-prone, as one only has to change the value once, at
the variable definition site, and the change will propagate to all uses of
that variable automatically.
<h3 id=placement><span class=secno>1.1. </span> Module Interactions</h3>
<p class=replaceme>Explain, normatively, how this module affects the
definition of CSS.
<p>This module replaces and extends the <var class=replaceme>SUMMARIZE
HERE</var> features defined in <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> sections <var
class=replaceme>W.X and Y.Z</var>.
<h3 id=values><span class=secno>1.2. </span> Values</h3>
<p>This specification follows the <a
href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
definition conventions</a> from <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 <a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Other CSS
modules may expand the definitions of these value types: for example <a
href="#CSS3COLOR" rel=biblioentry>[CSS3COLOR]<!--{{CSS3COLOR}}--></a>,
when combined with this module, expands the definition of the
<color> value type as used in this specification.
<h2 id=defining-variables><span class=secno>2. </span> Defining Variables</h2>
<p>A ‘<code class=css>@var</code>’ rule defines a variable and
assigns a value to it. It consists of the at-keyword ‘<code
class=css>@var</code>’ followed by a variable name (an identifier
starting with the "$" character), followed by the value and finally
terminated with a semi-colon. The result of this rule is that the given
value is assigned to the given variable name.
<p>
<p class=note>I chose the $ prefix over the var() function for terseness.
We should try to avoid nesting functions as much as reasonable, as it's
hard to read. The $ is required in the variable definition as well as the
variable use for consistency - every instance of the variable name within
CSS has the same form.
<div class=example>
<p>This variables declaration:</p>
<pre>@var $header-color #06c;</pre>
<p>declares a variable named "$header-color", and assigns to it the value
"#06c". This can then be used in any place where that value would be
valid, such as:
<pre>h1 { background-color: $header-color; }</pre>
<p>The preceding rule is equivalent to writing ‘<code
class=css>background-color: #06c;</code>’, except that the variable
name makes the origin of the color clearer, and if $header-color is used
elsewhere in the stylesheet, all of the uses can be updated at once by
changing the variable declaration.</p>
</div>
<p>Defined variables are available to all stylesheets in the document.
Thus, using ‘<code class=css>@import</code>’ to include a
stylesheet also includes all variables defined in the stylesheet (and
makes all variables declared outside available within the stylesheet, if
applicable). ‘<code class=css>@var</code>’ rules present in
disabled or alternate stylesheets do not define variables. Scoped
stylesheets are an exception to this: variables defined in the global
scope are available within a scoped stylesheet, but variables defined or
imported within scoped stylesheets are only available within the scoped
stylesheet and any imported stylesheets.
<p class=note>Making all variables global is the simplest solution that
works, and is consistent with other at-rules that define names, like
@font-face and @counter-style - the web seems to get by great on "ad-hoc
scoping" via adding short prefixes. If it is shown that we need a more
complex scoping solution in the future, I believe we can do so via a
general scoping/namespacing mechanism that applies to all the at-rules
that define names.
<p>A ‘<code class=css>@var</code>’ rule is <dfn
id=inactive-var-rule>inactive</dfn> if it's present in a disabled or
alternate stylesheet, a stylesheet with a media query that evaluates to
false, or is present within a conditional rule group [[CSS3CONDITIONAL]]
whose condition evaluates to false. <a
href="#inactive-var-rule"><i>Inactive</i></a> ‘<code
class=css>@var</code>’ rules do not define variables. All other
‘<code class=css>@var</code>’ rules are <dfn
id=active-var-rule>active</dfn>.
<p>If new ‘<code class=css>@var</code>’ rules are added to the
document through any means, such as direct CSSOM manipulation or new
stylesheets being added to the document, or if an <a
href="#inactive-var-rule"><i>inactive</i></a> ‘<code
class=css>@var</code>’ rule becomes <a
href="#active-var-rule"><i>active</i></a>, they define variables.
Similarly, if ‘<code class=css>@var</code>’ rules are removed
from the document or become <a
href="#inactive-var-rule"><i>inactive</i></a>, they no longer define
variables.
<p>If multiple <a href="#active-var-rule"><i>active</i></a> ‘<code
class=css>@var</code>’ rules attempt to define variables with the
same name, the last one defined wins; the preceding ‘<code
class=css>@var</code>’ rules do not define a variable. For this
purpose, user-agent defined rules come before author-defined rules, which
come before user-defined rules. Within each of the preceding groups, the
ordering is document order.
<div class=example>
<p>Earlier declarations of the same variable have no effect, even if uses
of the variable come between the earlier and final attempts to define the
variable:</p>
<pre>
@var $var red;
p { color: $var; }
@var $var blue;</pre>
<p>In this example, the color of <p> elements is blue, because the
second declaration wins over the first declaration in all uses.</p>
</div>
<p>Variables can refer to other variables in their value. If a dependency
cycle is created, all the declarations that directly contribute to the
cycle define <a href="#invalid-variable"><i>invalid variables</i></a>.
<div class=example>
<p>For example, the following set of variable declarations create a
dependency cycle:</p>
<pre>
@var $foo red;
@var $bar linear-gradient(transparent, $foo);
@var $foo $bar;</pre>
<p>The last two declarations create the cycle, and so they define
‘<code class=css>$bar</code>’ and ‘<code
class=css>$foo</code>’, respectively, as <a
href="#invalid-variable"><i>invalid variables</i></a> instead of as the
values in the declarations. If the last rule were removed, however, there
would be no problem - ‘<code class=css>$foo</code>’ would be
defined as ‘<code class=css>red</code>’, while ‘<code
class=css>$bar</code>’ would be defined as ‘<code
class=css>linear-gradient(transparent, red)</code>’.</p>
</div>
<p>Before they are defined, all variables are <a
href="#invalid-variable"><i>invalid variables</i></a>.
<h2 id=using-variables><span class=secno>3. </span> Using Variables</h2>
<p>A variable can be used anywhere a value is expected in CSS. Variables
can not be used as property names, selectors, or anything else besides
property values - doing so either produces an invalid value or, in some
situations like the attribute value of an attribute selector, a valid
value that nonetheless has no relation to the variable of that name.
<p>A variable is substituted for its value in the property value at
computed-value time. If a declaration, once all variables are substituted
in, is invalid, the declaration is <a
href="#invalid-at-computed-value-time"><i>invalid at computed-value
time</i></a>.
<div class=example>
<p>For example, the following usage is fine from a syntax standpoint, but
results in nonsense when the variable is substituted in:</p>
<pre>
@var $looks-valid 20px;
p { background-color: $looks-valid; }</pre>
<p>Since ‘<code class=css>20px</code>’ is an invalid value for
‘<code class=property>background-color</code>’, this instance
of the property computes to ‘<code
class=property>transparent</code>’ (the initial value for
‘<code class=property>background-color</code>’) instead.</p>
</div>
<h3 id=using-invalid-variables><span class=secno>3.1. </span> Using Invalid
Variables</h3>
<p>An <dfn id=invalid-variable title="invalid variable|invalid
variables">invalid variable</dfn> results from having variables directly
or indirectly refer to themselves, or from using an undefined variable.
Using an <a href="#invalid-variable"><i>invalid variable</i></a> in a
property value makes the declaration <a
href="#invalid-at-computed-value-time"><i>invalid at computed-value
time</i></a>.
<p>A declaration that is <dfn id=invalid-at-computed-value-time>invalid at
computed-value time</dfn> results from either using an <a
href="#invalid-variable"><i>invalid variable</i></a> in a property value,
or using a valid variable that produces an invalid declaration when it is
substituted in. When this happens, the declaration must compute to the
property's initial value.
<div class=example>
<p>For example, in the following code:</p>
<pre>
@var $invalid $invalid; /* Invalid, due to the trivial cycle. */
p { background-color: red; }
p { background-color: $invalid; }</pre>
<p>the <p> elements will have transparent backgrounds (the initial
value for ‘<code class=property>background-color</code>’),
rather than red backgrounds.</p>
<p>Note the difference between this and what happens if the author had
just written ‘<code class=css>background-color: 20px</code>’
directly in their stylesheet - that would be a normal syntax error, which
would cause the rule to be discarded, so the ‘<code
class=css>background-color: red</code>’ rule would be used instead.</p>
</div>
<p class=note>The <a href="#invalid-at-computed-value-time"><i>invalid at
computed-value time</i></a> concept exists because variables can't "fail
early" like other syntax errors can, so by the time the user agent
realizes a property value is invalid, it's already thrown away the other
cascaded values. I think ‘<code class=css>attr()</code>’ needs
to rely on it as well, as its behavior is almost identical to variables.
<h2 id=cssom><span class=secno>4. </span> APIs</h2>
<p>CSS Variables are mutable - one can change them after they've been
defined, through the CSSOM. This can be done in two ways: one can read the
current variable definition and set an override definition through the
convenient <code>.vars</code> property, or manipulate the definitions in
the stylesheets directly through the standard CSSOM stylesheet interface.
<h3 id=cssom-simple><span class=secno>4.1. </span> The Simple API</h3>
<p class=issue>Define the more convenient CSSOM API from <a
href="http://www.xanthir.com/blog/b4AD0">my blog post</a>.
<h3 id=cssom-stylesheet><span class=secno>4.2. </span> Additions to the
Stylesheet Interface</h3>
<p>This specification extends the IDL definitions in the CSSOM spec <a
href="#CSSOM" rel=biblioentry>[CSSOM]<!--{{!CSSOM}}--></a> in several
ways.
<h4 id=cssom-cssrule><span class=secno>4.2.1. </span> Changes to interface
CSSRule</h4>
<dl>
<dt>IDL Definition
<dd>
<pre class=idl>
interface CSSRule {
// RuleType
const unsigned short UNKNOWN_RULE = 0;
const unsigned short STYLE_RULE = 1;
const unsigned short CHARSET_RULE = 2;
const unsigned short IMPORT_RULE = 3;
const unsigned short MEDIA_RULE = 4;
const unsigned short FONT_FACE_RULE = 5;
const unsigned short PAGE_RULE = 6;
<ins>const unsigned short <a href="#variablerule"><code>VARIABLE_RULE</code></a> = 11;</ins>
readonly attribute unsigned short type;
attribute DOMString cssText;
// raises(DOMException) on setting
readonly attribute CSSStyleSheet parentStyleSheet;
readonly attribute CSSRule parentRule;
};</pre>
<dt>Defined Constants
<dd><dfn id=variablerule><code>VARIABLE_RULE</code></dfn>: The rule is a
<code>CSSVariableRule</code>.
</dl>
<p class=issue>Going with value 11 for now, since CSSOM seems to reserve
0-10.
<h4 id=cssom-cssvariablerule><span class=secno>4.2.2. </span> Interface
CSSVariableRule</h4>
<p>The <code>CSSVariableRule</code> interface represents a ‘<code
class=css>@var</code>’ rule within a CSS stylesheet. The
‘<code class=css>@var</code>’ rule is used to define
variables.
<dl>
<dt>IDL Definition
<dd>
<pre class=idl>
interface CSSVariableRule : CSSRule {
attribute DOMString name;
attribute DOMString value;
}</pre>
</dl>
<h4 id=cssom-cssvariable><span class=secno>4.2.3. </span> Interface
CSSVariableComponentValue</h4>
<p>The CSSVariableComponentValue interface represents a call to a CSS
Variable.
<dl>
<dt>IDL Definition
<dd>
<pre class=idl>
[NoInterfaceObject] interface CSSVariableComponentValue {
attribute DOMString variableName;
readonly attribute any variableValue;
}</pre>
<dt>Attributes
<dd>
<dl>
<dt><code>variableName</code> of type <code>DOMString</code>
<dd>This attribute is used for the name of the variable. Changing this
attribute changes the variable being referred to.
<dt><code>variableValue</code> of type <code>any</code>, readonly
<dd>This attribute is used for the value of the variable.
</dl>
</dl>
<h2 id=grammar><span class=secno>5. </span> The Grammar of Variables</h2>
<p class=issue>I'm not sure if I've done this section correctly. For now,
I'll try my best to copypasta what Conditionals is doing, since dbaron
usually know what's what.
<p>This specification extends the lexical scanner in the <a
href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, Appendix G) by
adding:
<pre>
@{V}{A}{R} {return VARIABLE_SYM;}
"$"name {return VARIABLE;}</pre>
<p>and the grammar by adding:
<pre>
variable_rule
: VARIABLE_SYM S+ variable_name S+ expr ':';
;</pre>
<p>and by amending:
<pre>
stylesheet
: [ CHARSET_SYM STRING ';' ]?
[S|CDO|CDC]* [ import [ CDO S* | CDC S* ]* ]*
[ [ ruleset | media | page <ins>| variable_rule</ins> ] [ CDO S* | CDC S* ]* ]*
;
nested_statement
: ruleset | media | page | font_face_rule | keyframes-rule |
supports_rule | document_rule <ins>| variable_rule</ins>
;
term
: unary_operator?
[ NUMBER S* | PERCENTAGE S* | LENGTH S* | EMS S* | EXS S* | ANGLE S* |
TIME S* | FREQ S* ]
| STRING S* | IDENT S* | URI S* | hexcolor | function <ins>| VARIABLE S*</ins>
;</pre>
<p class=issue>This uses the ‘<code
class=css>nested_statement</code>’ production from Conditionals.
That should make its way to a proper draft, like a new release of Syntax.
<p class=issue>Should variables be usable elsewhere, like in the value of a
MQ?
<h2 id=conformance><span class=secno>6. </span> Conformance</h2>
<h3 id=conventions><span class=secno>6.1. </span> Document Conventions</h3>
<p>Conformance requirements are expressed with a combination of descriptive
assertions and RFC 2119 terminology. The key words “MUST”, “MUST
NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
“SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the
normative parts of this document are to be interpreted as described in RFC
2119. However, for readability, these words do not appear in all uppercase
letters in this specification.
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a
href="#RFC2119" rel=biblioentry>[RFC2119]<!--{{!RFC2119}}--></a>
<p>Examples in this specification are introduced with the words “for
example” or are set apart from the normative text with
<code>class="example"</code>, like this:
<div class=example>
<p>This is an example of an informative example.</p>
</div>
<p>Informative notes begin with the word “Note” and are set apart from
the normative text with <code>class="note"</code>, like this:
<p class=note>Note, this is an informative note.
<h3 id=conformance-classes><span class=secno>6.2. </span> Conformance
Classes</h3>
<p>Conformance to CSS Variables Module is defined for three conformance
classes:
<dl>
<dt><dfn id=style-sheet title="style sheet!!as conformance class">style
sheet</dfn>
<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
style sheet</a>.
<dt><dfn id=renderer>renderer</dfn>
<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
that interprets the semantics of a style sheet and renders documents that
use them.
<dt><dfn id=authoring-tool>authoring tool</dfn>
<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
that writes a style sheet.
</dl>
<p>A style sheet is conformant to CSS Variables Module if all of its
declarations that use properties defined in this module have values that
are valid according to the generic CSS grammar and the individual grammars
of each property as given in this module.
<p>A renderer is conformant to CSS Variables Module if, in addition to
interpreting the style sheet as defined by the appropriate specifications,
it supports all the features defined by CSS Variables Module by parsing
them correctly and rendering the document accordingly. However, the
inability of a UA to correctly render a document due to limitations of the
device does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
<p>An authoring tool is conformant to CSS Variables Module if it writes
style sheets that are syntactically correct according to the generic CSS
grammar and the individual grammars of each feature in this module, and
meet all other conformance requirements of style sheets as described in
this module.
<h3 id=partial><span class=secno>6.3. </span> Partial Implementations</h3>
<p>So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers <strong>must</strong> treat as
invalid (and <a
href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore as
appropriate</a>) any at-rules, properties, property values, keywords, and
other syntactic constructs for which they have no usable level of support.
In particular, user agents <strong>must not</strong> selectively ignore
unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid (as
unsupported values must be), CSS requires that the entire declaration be
ignored.
<h3 id=experimental><span class=secno>6.4. </span> Experimental
Implementations</h3>
<p>To avoid clashes with future CSS features, the CSS2.1 specification
reserves a <a
href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
syntax</a> for proprietary and experimental extensions to CSS.
<p>Prior to a specification reaching the Candidate Recommendation stage in
the W3C process, all implementations of a CSS feature are considered
experimental. The CSS Working Group recommends that implementations use a
vendor-prefixed syntax for such features, including those in W3C Working
Drafts. This avoids incompatibilities with future changes in the draft.
<h3 id=testing><span class=secno>6.5. </span>Non-Experimental
Implementations</h3>
<p>Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they can
demonstrate to be correctly implemented according to spec.
<p>To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental CSS
renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before releasing
an unprefixed implementation of any CSS features. Testcases submitted to
W3C are subject to review and correction by the CSS Working Group.
<p>Further information on submitting testcases and implementation reports
can be found from on the CSS Working Group's website at <a
href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
Questions should be directed to the <a
href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
mailing list.
<h3 id=cr-exit-criteria><span class=secno>6.6. </span> CR Exit Criteria</h3>
<p class=issue>[Change or remove the following CR exit criteria if the spec
is not a module, but, e.g., a Note or a profile. This text was <a
href="http://www.w3.org/Style/CSS/Tracker/actions/44"> decided on
2008-06-04.</a>]
<p> For this specification to be advanced to Proposed Recommendation, there
must be at least two independent, interoperable implementations of each
feature. Each feature may be implemented by a different set of products,
there is no requirement that all features be implemented by a single
product. For the purposes of this criterion, we define the following
terms:
<dl>
<dt>independent
<dd>each implementation must be developed by a different party and cannot
share, reuse, or derive from code used by another qualifying
implementation. Sections of code that have no bearing on the
implementation of this specification are exempt from this requirement.
<dt>interoperable
<dd>passing the respective test case(s) in the official CSS test suite,
or, if the implementation is not a Web browser, an equivalent test. Every
relevant test in the test suite should have an equivalent test created if
such a user agent (UA) is to be used to claim interoperability. In
addition if such a UA is to be used to claim interoperability, then there
must one or more additional UAs which can also pass those equivalent
tests in the same way for the purpose of interoperability. The equivalent
tests must be made publicly available for the purposes of peer review.
<dt>implementation
<dd>a user agent which:
<ol class=inline>
<li>implements the specification.
<li>is available to the general public. The implementation may be a
shipping product or other publicly available version (i.e., beta
version, preview release, or “nightly build”). Non-shipping product
releases must have implemented the feature(s) for a period of at least
one month in order to demonstrate stability.
<li>is not experimental (i.e., a version specifically designed to pass
the test suite and is not intended for normal usage going forward).
</ol>
</dl>
<p>The specification will remain Candidate Recommendation for at least six
months.
<h2 class=no-num id=acknowledgments>Acknowledgments</h2>
<p><var class=replaceme>[acknowledgments]</var>
<h2 class=no-num id=references>References</h2>
<h3 class=no-num id=normative-references>Normative references</h3>
<!--begin-normative-->
<!-- Sorted by label -->
<dl class=bibliography>
<dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
<!---->
<dt id=CSS21>[CSS21]
<dd>Bert Bos; et al. <a
href="http://www.w3.org/TR/2011/REC-CSS2-20110607/"><cite>Cascading Style
Sheets Level 2 Revision 1 (CSS 2.1) Specification.</cite></a> 7 June
2011. W3C Recommendation. URL: <a
href="http://www.w3.org/TR/2011/REC-CSS2-20110607/">http://www.w3.org/TR/2011/REC-CSS2-20110607/</a>
</dd>
<!---->
<dt id=CSSOM>[CSSOM]
<dd>Anne van Kesteren. <a
href="http://www.w3.org/TR/2011/WD-cssom-20110712/"><cite>CSSOM.</cite></a>
12 July 2011. W3C Working Draft. (Work in progress.) URL: <a
href="http://www.w3.org/TR/2011/WD-cssom-20110712/">http://www.w3.org/TR/2011/WD-cssom-20110712/</a>
</dd>
<!---->
<dt id=RFC2119>[RFC2119]
<dd>S. Bradner. <a href="http://www.ietf.org/rfc/rfc2119.txt"><cite>Key
words for use in RFCs to Indicate Requirement Levels.</cite></a> Internet
RFC 2119. URL: <a
href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a>
</dd>
<!---->
</dl>
<!--end-normative-->
<h3 class=no-num id=other-references>Other references</h3>
<!--begin-informative-->
<!-- Sorted by label -->
<dl class=bibliography>
<dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
<!---->
<dt id=CSS1>[CSS1]
<dd>Håkon Wium Lie; Bert Bos. <a
href="http://www.w3.org/TR/2008/REC-CSS1-20080411"><cite>Cascading Style
Sheets (CSS1) Level 1 Specification.</cite></a> 11 April 2008. W3C
Recommendation. URL: <a
href="http://www.w3.org/TR/2008/REC-CSS1-20080411">http://www.w3.org/TR/2008/REC-CSS1-20080411</a>
</dd>
<!---->
<dt id=CSS3COLOR>[CSS3COLOR]
<dd>Tantek Çelik; Chris Lilley; L. David Baron. <a
href="http://www.w3.org/TR/2011/REC-css3-color-20110607/"><cite>CSS Color
Module Level 3.</cite></a> 7 June 2011. W3C Recommendation. URL: <a
href="http://www.w3.org/TR/2011/REC-css3-color-20110607/">http://www.w3.org/TR/2011/REC-css3-color-20110607/</a>
</dd>
<!---->
</dl>
<!--end-informative-->
<h2 class=no-num id=index>Index</h2>
<!--begin-index-->
<ul class=indexlist>
<li>active, <a href="#active-var-rule"
title=active><strong>2.</strong></a>
<li>authoring tool, <a href="#authoring-tool" title="authoring
tool"><strong>6.2.</strong></a>
<li>inactive, <a href="#inactive-var-rule"
title=inactive><strong>2.</strong></a>
<li>invalid at computed-value time, <a
href="#invalid-at-computed-value-time" title="invalid at computed-value
time"><strong>3.1.</strong></a>
<li>invalid variable, <a href="#invalid-variable" title="invalid
variable"><strong>3.1.</strong></a>
<li>invalid variables, <a href="#invalid-variable" title="invalid
variables"><strong>3.1.</strong></a>
<li>renderer, <a href="#renderer" title=renderer><strong>6.2.</strong></a>
<li>style sheet
<ul>
<li>as conformance class, <a href="#style-sheet" title="style sheet, as
conformance class"><strong>6.2.</strong></a>
</ul>
<li><a href="#variablerule"><code>VARIABLE_RULE</code></a>, <a
href="#variablerule" title="VARIABLE_RULE"><strong>4.2.1.</strong></a>
</ul>
<!--end-index-->
<h2 class=no-num id=property-index>Property index</h2>
<!--begin-properties-->
<table class=proptable>
<thead>
<tr>
<th>Property
<th>Values
<th>Initial
<th>Applies to
<th>Inh.
<th>Percentages
<th>Media
<tbody>
</table>
<!--end-properties-->
</html>
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